1. Field of the Invention
The present invention relates to an electric double layer capacitor and a process for its production. Particularly, it relates to an electric double layer capacitor whereby the internal resistance of the electric double layer capacitor can be made low, its capacitance density can be made high and its productivity can be satisfactorily maintained, and a process for producing such an electric double layer capacitor.
2. Discussion of the Background
An electric double layer capacitor is excellent in the power density and the long term reliability by charge/discharge cycles, and it is being used as a power source for hybrid electric cars, or as an emergency power source. In such an application as power sources, a high voltage at a level of a few hundreds V is required.
Usually, the operating voltage of a unit cell of an electric double layer capacitor is relatively low (at a level of up to 2.6 V). Accordingly, from a few tens to a few hundreds of such unit cells are used as connected in series to form an electric double layer module.
As the structure of such a unit cell, a rectangular cell or a cylindrical cell is common.
A perspective view illustrating the structure of a rectangular cell (partially cut) is shown in FIG. 7.
As shown in FIG. 7, a rectangular cell 20A is one wherein a plurality of flat plate positive electrodes 1A and negative electrodes 1B are alternately stacked with a separator 2 interposed therebetween to form a rectangular element assembly 3, which is contained in a rectangular casing 5.
Further, from positive electrodes 1B and negative electrodes 1B, flat plate lead portions 7A and 7B extend upwardly, respectively, and they are respectively bundled into lead connecting portions 8A and 8B as divided into a positive electrode and a negative electrode. The lead connecting portions 8A and 8B are connected to a positive electrode terminal 9A and a negative electrode terminal 9B passing through and fixed to the rectangular casing 5.
On the other hand, a perspective view illustrating the structure of a cylindrical cell (partially cut) is shown in FIG. 8.
As shown in FIG. 8, a cylindrical cell 20B is constituted in such a manner that a pair of long strip-shaped positive electrode 11A and negative electrode 11B are wound up with a separator 12 interposed therebetween to form a wound element assembly 13, which is contained in a cylindrical casing 15.
Further, leads 17A and 17B are connected to the upper ends of the positive electrode 11A and negative electrode 11B, and these leads 17A and 17B are, respectively, connected to a positive electrode terminal 19A and a negative electrode terminal 19B passing through and fixed to a sealing insulation plate 16.
Unit cells 20A and 20B thus constituted, are, respectively, designed so that, for example, a plurality of them are connected in series to constitute an electric double layer module.
A perspective view illustrating an embodiment of such an electric double layer module structure, is shown in FIG. 9.
As shown in FIG. 9, the electric double layer module 25 is constituted by solid module structural members 21 to integrally secure a plurality of unit cells 20 (cylindrical cells 20B) and many connecting bus bar members 23 to electrically connect the unit cells 20 in series.
Further, another structure of an electric double layer module 25 may be one as shown in JP-A-2002-353078 wherein electrodes constituting the positive electrode 11A and the negative electrode 11B, etc. are specially designed so that the cylindrical casing 15 of unit cells 20, the module structural members 21 and the connecting bus bar members 23 are integrated to make the electric double layer module 25 compact and light in weight.
Of such a electric double layer capacitor for large capacitance and large current charging/discharging, it is desired to further reduce the internal resistance and to increase the capacitance per unit volume (hereinafter referred to as the capacitance density).
Accordingly, it is conceivable to enlarge the surface area of the electrode and to reduce the thickness of the separator 2 or 12 as far as possible.
However, in such a case, the following structural problem is likely to result.
For example, the separator 2 or 12 is required to have the porosity set to be high to some extent from the viewpoint of absorption and retention of the electrolyte. Here, the porosity is the proportion of the volume occupied by voids (bubbles present in the object) in the volume of the object.
Therefore, if it is attempted to reduce the thickness of the separator 2 or 12, while maintaining the porosity to be high to some extent, the insulation between the positive electrode 11A and the negative electrode 11B tends to be inadequate, whereby the positive electrode 11A and the negative electrode 11B tends to undergo microscopic short circuiting, thus leading to self-discharge or a decrease in the production yield of the capacitor.
Further, if the thickness of the separator 2 or 12 is made too thin (e.g. at most 60 μm), it tends to be difficult to increase the porosity of the separator 2 or 12, whereby the amount of the electrolyte in the separator 2 or 12 tends to be small, whereby it tends to be difficult to supply the electrolyte to the positive electrode 11A and the negative electrode 11B sufficiently.
Consequently, no adequate electrolyte will be supplied to the positive electrode 11A and the negative electrode 11B, whereby no adequate amount of ions will be present in the vicinity of the positive electrode 11A and the negative electrode 11B, and at the time of discharge, the voltage drop is likely to be substantial due to instantaneous large current discharge.
Further, as no adequate electrolyte will be supplied to the positive electrode 11A and the negative electrode 11B, polarization of ions tends to be inadequate at the positive electrode 11A and the negative electrode 11B during the charge, whereby the voltage retention property is likely to be low. Further, no adequate adsorption of ions required for the external applied voltage for charging against the positive electrode 11A and the negative electrode 11B will be carried out, whereby an electrochemical decomposition reaction or the like other than the adsorption will take place at the positive electrode 11A and the negative electrode 11B, whereby the internal resistance is likely to increase, or the capacitance density is likely to decrease.
Further, if it is attempted to supply the electrolyte sufficiently to the positive electrode 11A and the negative electrode 11B to solve such problems, it takes time for injection of the electrolyte, thus leading to a problem in the productivity of the electric double layer capacitor.
As a method for supplying an electrolyte sufficiently to the electrodes, for example, JP-A-2001-44081 proposes a method wherein a groove is formed on the surface of the electrodes to maintain the electrolyte in the vicinity of the electrodes in an amount corresponding to the amount of the electrolyte expected to be dried up during the use.
However, this method is limited to maintain the electrolyte in an amount corresponding to the amount to be dried up, and, for example, in a case where the thickness of the separator 2 or 12 is made thin, no adequate amount of the electrolyte required for the polarization of ions, can be maintained in the separator 2 or 12.